linux-kernel - Re: [PATCH v1 1/2] tools/nolibc/stdlib: Support overflow checking for older compiler versions

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Message-ID: <20220520041915.GC5001@1wt.eu>
Date:   Fri, 20 May 2022 06:19:15 +0200
From:   Willy Tarreau <w@....eu>
To:     Ammar Faizi <ammarfaizi2@...weeb.org>
Cc:     "Paul E. McKenney" <paulmck@...nel.org>,
        Alviro Iskandar Setiawan <alviro.iskandar@...weeb.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        GNU/Weeb Mailing List <gwml@...r.gnuweeb.org>,
        Facebook Kernel Team <kernel-team@...com>
Subject: Re: [PATCH v1 1/2] tools/nolibc/stdlib: Support overflow checking
 for older compiler versions

Hi Ammar,

On Fri, May 20, 2022 at 12:21:15AM +0700, Ammar Faizi wrote:
> diff --git a/tools/include/nolibc/stdlib.h b/tools/include/nolibc/stdlib.h
> index 8fd32eaf8037..92378c4b9660 100644
> --- a/tools/include/nolibc/stdlib.h
> +++ b/tools/include/nolibc/stdlib.h
> @@ -128,10 +128,9 @@ void *malloc(size_t len)
>  static __attribute__((unused))
>  void *calloc(size_t size, size_t nmemb)
>  {
> -	void *orig;
> -	size_t res = 0;
> +	size_t x = size * nmemb;
>  
> -	if (__builtin_expect(__builtin_mul_overflow(nmemb, size, &res), 0)) {
> +	if (__builtin_expect(size && ((x / size) != nmemb), 0)) {

Ah, that approach is even better than mine, I'm seeing that on x86 the
compiler simply checks the overflow flag after the multiply, that's
perfect!

Acked-by: Willy Tarreau <w@....eu>

Willy